The desire to provide highly accurate, electronic reproductions of sound has led to the development of numerous designs for loudspeakers able to convert electronic energy into acoustic energy. Typically, a complete high fidelity audio system includes at least two loudspeakers, each of which is designed to produce sound over substantially the entire audio spectrum. Because the accurate reproduction of sound within different ranges of the audio spectrum may place conflicting constraints on loudspeaker design, each loudspeaker frequently includes a number of electromechanical drivers separately designed to output acoustic energy within predetermined frequency ranges. For example, in a three-way system, each loudspeaker includes three drivers for separately reproducing sounds in the low, middle, and high-frequency ranges of the audio spectrum. Alternatively, the high fidelity audio system may employ a plurality of loudspeakers separately designed to accurately reproduce sound within various frequency ranges.
One such loudspeaker, designed to reproduce sound at low or bass frequencies, is disclosed in U.S. Pat. No. 4,567,959. The disclosed loudspeaker includes a loudspeaker transducer or driver secured to a first, open end of an elongate tube. The end of the tube opposite the driver is closed by a flat wall. A port is provided through the tube wall adjacent the closed end of the tube and is covered by a bass-reflex duct, connected to and extending along the length of the tube. The end of the duct that is adjacent the closed end of the tube is also closed, while the end of the duct adjacent the loudspeaker driver is open. When electric energy is applied to the driver, acoustic energy is directly radiated from the outwardly facing surface of the driver cone. Acoustic energy developed by the inner surface of the driver cone, on the other hand, is primarily transmitted progressively through the tube, port, and duct. Because this supplemental energy reaches the listener's environment at the open end of the duct, the inclusion of the port and duct allows electric energy to be converted into acoustic energy in a more efficient manner than a closed-tube design.
The loudspeaker disclosed in U.S. Pat. No. 4,567,959 is intended to be acoustically loaded by placing the end of the loudspeaker including the driver and open duct approximately three inches from one of the facing walls of a corner. This placement allows the loudspeaker to provide a bass response that is enhanced in comparison to an ordinary free-standing acoustic suspension or bass-reflex speaker. Because of the manner in which the speaker is acoustically loaded, it is important that the audio energy emanate from only the plane of the driver and duct opening.
Although the loudspeaker design described above does provide an enhanced response at bass frequencies, it is not without several problems. First, the acoustic pressure variations established by the driver cone within the tube are typically sufficient to cause the flat, closed end of the tube to vibrate. As a result, spurious resonances are created at the lower audible frequencies, producing undesirable irregularities in the frequency response of the loudspeaker. While these resonances can be minimized by increasing the rigidity of the tube, correction in this manner may add significantly to the weight and expense of the loudspeaker.
A second disadvantage of the loudspeaker design described above is that the sound originates from one plane. While this allows the loudspeaker to be corner loaded more easily, it may impair the authenticity of a high fidelity reproduction when the sounds being reproduced did not originally come from a point source.
Finally, U.S. Pat. No. 4,567,959 describes the disclosed loudspeaker design as having particular applicability to the interior of a vehicle. As will be appreciated, due to the frequent changes in velocity experienced by a moving vehicle, a tubular loudspeaker having its longitudinal axis placed normal to the direction of travel will tend to roll when the vehicle accelerates or decelerates. Thus, some means for securing the loudspeaker in place is required. This necessity is complicated by the fact that most of the surface of the tube that can be used to make the attachment is curved.
In light of the foregoing considerations, it would be desirable to produce a loudspeaker having an enhanced bass frequency response that eliminates spurious resonances, emits sound from more than one plane, and that can be stably situated in an environment undergoing motional changes.